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Ubuntu 9.10 uses Grub2 that has different commands with grub1, this paper tells how to boot previous installed Linux and Xen that used to use Grub1. It shows also Grub2/Grub1 commend syntax differences.

1. I partitioned for CentOS5.2 and Ubuntu10.04 when I have not known I
can use single /boot. After installed CentOS and it works, I installed Ubuntu with
its own /boot and /. I can't boot my PC thus reinstalled grub into MBR using Ubuntu live
DVD. This results in bootable Ubuntu only. My partitions are:
sda1(CentOS /boot), swap, sda3(CentOS /), sda5(Ubuntu /boot), sda6(Ubuntu /), sda7(common data)

2. Ozar and yancek tells me Ubuntu 9.10+ uses Grub2... In Grub1 or CentOS' /dev/sda1/grub/menu.lst I have:
title CentOS (2.6.18-92.1.13.el5xen)
        root (hd0,0)
        kernel /xen.gz-2.6.18-92.1.13.el5
        module /vmlinuz-2.6.18-92.1.13.el5xen ro root=LABEL=/12 xencons=tty6 rhgb quiet
        module /initrd-2.6.18-92.1.13.el5xen.img
title CentOS (2.6.18-92.1.13.el5)
        root (hd0,0)
        kernel /vmlinuz-2.6.18-92.1.13.el5 ro root=LABEL=/12 rhgb quiet
        initrd /initrd-2.6.18-92.1.13.el5.img

where label /12 can also be seen via:
$ blkid /dev/sda3
/dev/sda3: LABEL="/12" UUID="56a49751-f9d1-4d86-9de4-73bd49f5af20" SEC_TYPE="ext2" TYPE="ext3"

3. as I use grub2 under Ubuntu I need to add previous CentOS entries into /etc/grub.d/40_custom:
menuentry 'CentOS 5.2, with Linux 2.6.18-92.1.13.xen' --class gnu-linux --class gnu --class os {
        insmod ext2
        set root='(hd0,1)'
        multiboot /xen.gz-2.6.18-92.1.13.el5
        module /vmlinuz-2.6.18-92.1.13.el5xen ro root=LABEL=/12 xencons=tty6 rhgb quiet
        module /initrd-2.6.18-92.1.13.el5xen.img
menuentry 'CentOS 5.2, with Linux 2.6.18-92.1.13' --class gnu-linux --class gnu --class os {
        insmod ext2
        set root='(hd0,1)'
        linux   /vmlinuz-2.6.18-92.1.13.el5 root=LABEL=/12 rhgb ro quiet splash
        initrd  /initrd-2.6.18-92.1.13.el5.img
Note Grub2 doesn't support "kernel" but "linux", and you must use "multiboot" for xen now. Also I can only use "root=LABEL=" syntax rather then Ubuntu's syntax - sth like that that uses "search" and "uuid" as below. Even "root=/dev/=sda3" does not work for me.
menuentry 'Ubuntu, with Linux 2.6.32-22-generic-pae' --class ubuntu --class gnu-linux --class gnu --class os {
        insmod ext2
        set root='(hd0,5)'
        search --no-floppy --fs-uuid --set 3e60d438-f679-4846-8ea1-92425cdf9c49
        linux   /vmlinuz-2.6.32-22-generic-pae root=/dev/sda6 ro   quiet splash
        initrd  /initrd.img-2.6.32-22-generic-pae

4. Disable uuid passing by edit /etc/default/grub, un-comment this line

5. Update and verify your additions:
$sudo update-grub

$sudo cat /boot/grub/grub.cfg

$sudo reboot

In this way I can boot previous installed Linux and Xen, along with new installed Ubuntu.

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